Kitchen hoods vent out either an exterior side wall or through the roof. Whether the vent exits a wall or the roof depends on the specific layout of the home.
When feasible, the most direct ventilation for kitchen hoods is through the side wall of the home without any vertical piping. Where the layout of the kitchen does not allow direct horizontal venting, a vertical vent, usually through cabinetry, is used to reach either the roof or an accessible exterior wall. Hoods come manufactured with knock-out sections both on the top and back to allow for vertical or horizontal venting.